1. Field of the Invention
This invention relates to improvements in a weft picking system for a fluid jet loom, and more particularly to such a weft picking system including a roller type weft traction device between a weft measuring and storing device and a fluid jet nozzle to draw a weft yarn at a high speed toward the fluid jet nozzle.
2. Description of the Prior Art
Fluid jet looms are provided with a weft picking system which is arranged to accomplish a weft picking under the influence of fluid jet ejected from a nozzle or nozzles. This weft picking system accomplishes weft picking at a high speed with the fluid jet and therefore is highly advantageous from the view point of improving productivity of a fabric and availability of the loom. However, for example, in case of accomplishing the weft picking under the influence of air jet, a weft yarn is pulled and flown under a frictional force between air stream and the weft yarn, and therefore such a weft picking system induces large pressurized air consumption (or electric power consumption), and is problematic in energy consumption.
In view of this, a weft picking system has been proposed to deal with the above energy consumption problem as disclosed, for example, in Japanese Patent Provisional Publication No. 57-199841. In this proposition, a weft traction device including a pair of rotating rollers are used, in which the rollers are contactable and separable so that a weft yarn is held between and drawn by the rollers particularly during the initial stage of a weft picking period at which time pressurized air consumption in a fluid jet nozzle is particularly high. Such a weft traction device functions to positively feed out the weft yarn under the action of the rollers driven at a high speed, and therefore the fluid jet nozzle such as a weft inserting or weft posture regulating nozzle is required merely to regulate the posture of the weft yarn to be projected into the shed of warp yarns, thereby saving air consumption and therefore energy consumption.
Thus, although the above-discussed conventional weft picking system is considerably effective from the view point of reducing energy consumption because the fluid jet nozzle is used only for regulating the posture of the weft yarn to be picked by virue of the direct traction of the rollers, drawbacks have been encountered in the conventional weft picking system as discussed below.
The weft traction device including the rollers are arranged to draw and push the weft yarn by the rollers (between which the weft yarn is put) rotating at a high speed during the weft picking period. Accordingly at the initial stage of the weft picking, a tractive force or speed (to the weft yarn) of the rollers is higher than that of a weft picking nozzle or weft posture regulating nozzle. Owing to this difference in tractive force (speed), a slackened portion of the weft yarn is unavoidably formed between the weft traction device and the weft posture regulating nozzle, and supplied to the weft posture regulating nozzle. In case that the inlet of the weft posture regulating nozzle is clogged with the weft yarn slackened portion, weft picking has failed. In case that the weft yarn slackened portion passes through the weft posture regulating nozzle, the slackened portion is left as it is in a woven cloth thereby forming a weaving defect. Particularly in case of using highly twisted weft yarns, flection of the slackened portion is promoted thereby making an entangled state of the weft yarn. This is left as a defect in the woven fabric thus forming a so-called kink.
Causes of the above weft picking failures and weaving defects will be discussed with reference to FIGS. 34 and 35A to 35C.
In a weft picking, first the weft posture regulating nozzle begins to draw the weft yarn. Subsequently, the rollers of the weft traction device begins to draw the weft yarn. This is illustrated in the graph of FIG. 34 in which the abscissa represents the lapsed time while the ordinate represents the tractive speed (the speed of the drawn weft yarn). A curve a represents the change in tractive speed of the weft posture regulating nozzle while a line b represents the change in tractive speed of the weft traction device.
Detailed explanation will be made on the assumption that a sufficient distance (in length) is set between the weft traction device and the weft posture regulating nozzle. First, the weft posture regulating nozzle begins to eject air jet at a time t1, and then draw the weft yarn, gradually increasing its tractive speed. Subsequently, the rollers of the weft traction device begin to draw or push the weft yarn at a time t2. The traction speed of the weft traction device is momentarily raised in a state in which the weft yarn is kept between the rollers rotating at a constant high speed at which the tractive speed is higher than that of the weft posture regulating nozzle. Accordingly, until the tractive speed of the weft posture regulating nozzle reaches that of the weft traction device, the weft yarn slackened portion is being formed and increases speed. Thereafter, when the tractive speeds of the weft traction device and the weft posture regulating nozzle become the same level at a time t3, an increase of the weft yarn slackened portion stops, and then the slackened portion is gradually diminished as the tractive speed of the weft posture regulating nozzle becomes greater than that of the weft traction device. As discussed above, in case that the weft posture regulating nozzle is sufficiently spaced from the weft traction device, the weft yarn slackened portion is diminished and disappears before reaching the weft posture regulating nozzle, thus preventing the slackened portion from being introduced into the weft posture regulating nozzle.
However, it is to be noted that such a sufficient distance cannot be set between the weft traction device and the weft posture regulating nozzle in practice from the view point of rendering the whole loom compact. Consequently, in such a situation, the weft yarn slackened portion unavoidably reaches the weft posture regulating nozzle before the slackened portion disappears, thereby causing the problems of weft picking failure and weaving defects. The weft yarn slackened portion usually takes a form of FIGS. 35A, 35B or 35C and continuously extends along the moving direction of the weft yarn between the weft traction device and the weft posture regulating nozzle. The form of the weft yarn slackened portion changes depending on weft picking condition and/or kinds of a weft yarn used.
In addition to the above proposition in Japanese Patent Provisional Publication No. 57-199841, another proposition of a weft picking system is made to save air consumption as disclosed in Japanese Patent Provisional Publication No. 4-136237. In this proposition, a weft traction device includes rotatable rollers between a weft measuring and storing device and a fluid jet nozzle, and a weft yarn is kept between the rollers to be drawn and pushed toward the fluid jet nozzle as in the above-discussed proposition. The rotation of the rollers is controlled in timed relation to a loom main shaft by a servo motor, wherein the rollers rotate at a low speed to receive the weft yarn therebetween at the initial stage of the weft picking; the rollers rotate at a high speed to cause the weft yarn to fly at a high speed at the middle stage of the weft picking; and again the rollers rotate at the low speed to release the weft yarn at the terminal stage of the weft picking. Thus, this proposition is intended to prevent a slackened portion of the weft yarn from being formed, by suitably controlling the tractive speed or rotation speed of the rollers of the weft traction device.
However, the weft picking system of this proposition requires a precise rotational control of the rollers by using the servo motor and therefore is difficult to be put into practical use. More specifically, it is generally impossible to make such a follow-up control as to make a considerable change in rotation of the rollers for a very short time for each weft picking particularly when the rotation speed of the loom main shaft is high to meet a high speed loom operation. Even if put into practical use, it requires a very expensive control system which is high in response and control precision and a motor which can momentarily generate a high torque. Thus, the weft picking system of this proposition is difficult to be used in usual looms from a variety of view points.